lamB is the structural gene for the bacteriophage lambda receptor in Escherichia coli K-12. In vivo and in vitro studies of the lambda receptor from lamB missence mutants selected as resistant to phage lambda h+ showed the following. (i) Resistance was not due to a change in the amount of lambda receptor protein present in the outer membrane but rather to a change in activity. All of the mutants were still sensitive to phage lambda hh*, a two-step host range mutant of phage lambda h+. Some (10/16) were still sensitive to phage lambda h, a one-step host range mutant. (ii) Resistance occurred either by a loss of binding ability or by a block in a later irreversible step. Among the 16 mutations, 14 affected binding of lambda h+. Two (lamB106 and lamB110) affected inactivation but not binding; they represented the first genetic evidence for a role of the lambda receptor in more than one step of phage inactivation. Similarly, among the six mutations yielding resistance to lambda h, five affected binding and one (lamB109) did not. (iii) The pattern of interactions between the mutated receptors and lambda h+ and its host range mutants were very similar, although not identical, in vivo and in vitro. Defects were usually more visible in vitro than in vivo, the only exception being lamB109. (iv) The ability to use dextrins as a carbon source was not appreciably affected in the mutants. Possible working models and the relations between phage infection and dextrins transport were briefly discussed.